Polychromatic glasses, i.e., glasses wherein a full spectrum of colors can be secured in a single composition, had their genesis in U.S. Pat. No. 4.017,318. As disclosed therein, such glasses are produced through a series of irradiation and heat treating steps. A wide range of base glass compositions is operable but, as essential components, each glass must include silver, an alkali metal oxide which is preferably Na.sub.2 O, fluoride, and at least one halide selected from the group of chloride, bromide, and iodide. The glass articles are irradiated with high energy or actinic radiations selected from the group of high velocity electrons, X-radiations, and ultra-violet radiations, the latter generally having wavelengths within the range of about 2800A-3500A. The heat treatments involve subjecting the glass articles to temperatures between about the transformation range up to about the softening point thereof. Where ultra-violet radiation constitutes the effective actinic radiation, that patent indicates the presence of CeO.sub.2 to be necessary in the glass composition.
As is explained in that patent, the method for preparing such glasses comprehends seven general steps.
First, a glass-forming batch of an appropriate composition is melted.
Second, the molten batch is simultaneously cooled and shaped into a glass article of a desired configuration.
Third, the glass article is exposed to high energy or actinic radiations, this exposure effecting the development of a latent image in the glass. The time and intensity of this exposure determines the final color which will be produced in the glass.
Fourth, the so-exposed glass article is subjected to a heat treatment which promotes the precipitation of colloidal silver particles in situ to function as nuclei. Where a colored transparent glass is the goal, this heat treatment will be continued for only so long as to cause the precipitation of colloidal silver and to initiate the growth thereon of extremely small microcrystals of alkali metal fluoride-silver halide, e.g., NaF'(AgC1 and/or AgBr and/or AgI). Where a colored opal glass is desired, this heat treatment will be undertaken for a period of time sufficient to not only induce the precipitation of colloidal silver nuclei, but also to effect the growth of said microcrystals on the silver nuclei to a size large enough to scatter light. Hence, opacity will be developed in the exposed portions of the glass article with any unexposed portion remaining transparent.
Fifth, the nucleated glass article is called to a temperature at least 25.degree. C. below the strain point of the glass, optionally to ambient or room temperature.
Sixth, the cooled glass article is exposed to high energy or actinic radiations, this exposure intensifying the color which will subsequently be developed. (The color to be produced was determined in Step 3 above).
Seventh, the re-exposed glass article is again subjected to a heat treatment which produces the desired color in the glass. It was theorized that this heat treatment caused the precipitation of submicroscopic particles of silver, either as discrete colloidal particles and/or deposited on the surface and/or deposited within the alkali metal fluoride-silver halide microcrystals.
Although the mechanism underlying the development of color in such glasses has not been fully elucidated, it is hypothesized to be related to the quantity of silver precipitated and the geometry thereof, as well as, perhaps, the refractive index of the crystals. Furthermore, because the colors can be obtained with very low contents of silver and demonstrate characteristics similar to interference colors, it has been conjectured that at least one of the three following circumstances exists: (1) discrete colloidal silver particles less than about 200A in the smallest dimension; (2) metallic silver deposited within the alkali metal fluoride-silver halide microcrystals, the silver-containing portion of the microcrystals being less than about 200A in the smallest dimension; and (3) metallic silver deposited upon the surface of the microcrystals, the silver-coated portion of the microcrystals being less than about 200A in the smallest dimension.
Finally, the patent observed that the heat treatment after each exposure to high energy or actinic radiation could optionally be in the form of a series of heatings and coolings rather than a single treatment as set out above. Such additional heat treatments do not appear to substantially alter the color developed but can intensify the color produced.
U.S. Pat. No. 4,092,139 describes a modification of the method for preparing polychromatic glasses disclosed in U.S. Pat. No. 4,017,318, supra. That modification contemplated combining the exposure to high energy or actinic radiation and heat treatment into a single process. That is, the exposure is carried out while the glass article is at a temperature between about 200.degree.-410.degree. C. The inventive method is stated to be operable over the composition ranges recited in U.S. Pat. No. 4,017,318 and provides the advantage of producing a similar product while reducing the treatment time required to secure a colored glass and, at the same time, improving the intensity of the color developed.
The Food and Drug Administration (FDA) has ruled that all lenses sold in the United States for ophthalmic applications must pass a specified ball drop test, viz., a 5/8" diameter steel ball falling from a height of 50". To comply with this ruling has required that lenses formed from glass be strengthened in some manner. Two methods to achieve such strengthening have generally been practiced in the industry.
The first of these, termed air tempering or thermal tempering, involves heating the glass lens to a temperature approaching the softening point of the glass and then quickly chilling it, customarily through quenching in a stream of cool air. This practice is not available for use with polychromatic glasses because the colors therein are removed completely at temperatures approximating the strain point of the glasses.
The second method, variously called chemical strengthening or chemical tempering, comprehends an ion exchange reaction carried out at elevated temperatures but below the strain point of a glass. In the most common case, a glass containing alkali metal ions is contacted with a source of alkali metal ions (typically a bath of a molten salt) having a larger ionic radius than the alkali metal ions in the glass. Upon contact, the larger ions diffuse into the surface of the glass and replace the smaller alkali metal ions on an equal molar basis. Because the ion exchange is carried out at temperatures below the strain point of the glass, the larger ions are squeezed or stuffed into the sites originally occupied by the smaller ions. Since the glass structure at those temperatures is relatively firm, this crowding of the larger ions into the surface of the glass body causes compressive stresses to be developed in the glass surface, thereby imparting improved mechanical strength to the glass article.
The rate at which the ion exchange reaction takes place is directly dependent upon the temperature employed. Nevertheless, as the temperature closely approaches the strain point of the class, stress relaxation will occur in the surface of the article with consequent reduction in strength improvement. Accordingly, empirical testing will be undertaken to determine the temperature at which maximum surface compression can be developed (within a practical length of time) without significant stress release. Furthermore, as was noted above, the colors in polychromatic glasses are lost when exposed to temperatures approximating the strain points thereof.
Laboratory experience has determined that those glasses which develop an ion exchanged surface layer of at least 0.002" in depth, a maximum central tension of at least 0.7 kg/mm.sup.2, and a modulus of rupture approaching 30,000 psi will typically survive the ball drop text. An ion exchange reaction time of less than about 24 hours is preferred from a commercial point of view, although longer times may yield comparable or even superior strengths.
Where a lens is designed for prescription ophthalmic applications, the refractive index of the glass takes on critical significance. Hence, the standard refractive index utilized in glass prescription lenses is 1.523.+-.0.0003. This circumstance is due to the type of glasses presently in commercial use. A glass having a different refractive index would necessitate changes in the apparatus conventionally used by the lens manufacturer and, therefore, would not be a commercially saleable product.
The preferred embodiment recited in U.S. Pat. No. 4,017,318 consisted essentially, in weight percent on the oxide basis, of about 10-20% Na.sub.2 O, 0.0005-0.3% Ag, 1-4% F, an amount of at least one halide selected from the group consisting of Cl, Br, and I sufficient to react stoichiometrically with the Ag, but not more than a total of about 3%, and the remainder SiO.sub.2. Where ultra-violet radiation constitutes the actinic radiation, 0.01-0.2% CeO.sub.2 will be included in the composition. The patent notes the optional useful additions of up to 18% ZnO and/or up to 10% Al.sub.2 O.sub.3.